Computed radiographic tomography is a well-known method for analysing the internal structure of various objects, parts of objects and target objects. It has use in diverse fields, including medical imaging, product quality control, and component defect analysis.
The basic concept of computerised radiographic tomography is shown in FIG. 1. FIG. 1 shows a source of radiation S and a detector D for the same type of radiation. The source S emits a beam B of the radiation which is detected at detector D. Interposed between source S and detector D is a target object T, the internal structure of which is intended to be analysed. As beam B of radiation passes through target object T, it is impeded to a greater or lesser extent depending on the properties and interaction path length of the material of target object T which is intersected by beam B. Detector D registers the intensity of radiation arriving from source S via target object T, usually as a two-dimensional pixel image. The image acquired this way is a projection of the target object T in terms of its transparency to the radiation. Such an image represents a projective view of the object from a single direction, and is essentially equivalent to a conventional radiographic image. X-ray radiation is the conventional choice of radiation, but other forms of radiation whose intensity is attenuated to different degrees by different internal structures or compositions of matter may also be selected, without limitation.
In computed radiographic tomography, the target object T is relatively rotated around axis A with respect to a reference frame defined by source S, detector D, and beam B. At small angular intervals of rotation, typically one tenth or one hundredth of a degree, a sequence of radiographic projections is acquired by detector D. After a complete circular rotation about axis A, the sequence of images so obtained, for example 3,000 to 30,000 images, typically up to 10,000, are synthesized into a volume map of object T in terms of the relative opacity of target object T to the selected radiation. While it is conventional to use a complete 360 degree rotation to obtain the sequence of images, in some cases it is acceptable to acquire images covering a rotation angle of at least 180 degrees.
Such a volume map can be used to determine the internal structure of target object T. The mathematical techniques used to transform the series of individual radiographic images, or radiographic projections as they are conventionally termed, into the volume map form part of the common general knowledge of one skilled in the art in this field, and are normally computerised or computer-implemented.
Depending on the type, and especially on the size, of target object T, the fixed reference frame for the rotation about axis A can be selected. In some cases, the source and detector can in opposition rotate about the axis A, for example in medical imaging applications where it is impractical to rotate a human body or body part as a whole. In other cases, when target object T is small, the object may be placed on a turntable and can be rotated about axis A while using a fixed source S and fixed detector D. This latter scenario is usual in industrial CT imaging.
However, as in all mechanical systems, misalignments, tolerances, and mechanical inaccuracies can cause deviations from the ideal system shown in FIG. 1. For example, the source and detector might relatively move during exposure, the relative rotation of the target object and source may not be perfect about axis A, and vibration and other effects can generally contribute to a less than optimal imaging or reconstruction situation. Especially, the axis of rotation can wobble or precess, or the source can expand due to heating during the exposure. Reconstructing volume maps from such imperfect imaging scenarios will generally result in a loss of detail and/or blurring in the acquired volume map.
Such errors in the relative movement of source and target object are a principal impediment to the development of high-resolution computerised radiographic tomography techniques, such as may be used for the analysis, for example, of very small electronic components or for very precise analysis of the human body. Improving useful resolution of the reconstructed volume map is an important goal in this field. However, it is very challenging to improve the accuracy of the mechanisms which create the relative movement necessary for computerised radiographic tomography. Therefore, the present inventor has recognised the need for a technique to minimise the effect such undesired movement has on the reconstructed volume map, rather than concentrating on improving the mechanical limitations of the imaging apparatus.